Fluid density transducers

ABSTRACT

A fluid density transducer in the form of a single tube electromagnetically excited into second or higher overtone transverse vibration, the fluid, normally a liquid, flowing through the tube. Resonant vibration is maintained by feedback through an amplifier from an electromagnetic pickup. The frequency of the vibration is representative of the density of the fluid in the tube.

11/1953 Fay 3,444,723 5/1969 Wakefield... 73/32 3,449,941 6ll969 Banks 73/32 3,456,491 7/1969 Brockhaus 73/32 1 01113103: PATENTS 1,126,450 9/1968 Great Britain 73/32 Primary Examiner--Richard C. Queisser Assistant Examiner-C. E. Snee, III

Att0rneys-William R. Sherman, Stewart F. Moore, Jerry M.

Presson and Arnold, Roylance, Kruger & Durkee ABSTRACT: A fluid density transducer in the form of a single tube electromagnetically excited into second or higher overtone transverse vibration, the fluid, normally a liquid, flowing through the tube. Resonant vibration is maintained by feedback through an amplifier from an electromagnetic pickup.

The frequencyof the vibration is representative of the density of the fluid in the tube.

PATENTED JUN22|971 INVENTOR James Wdryche Sfansfeld ATTORNEY FLUID DENSITY TRANSDUCERS FIELD or THE INVENTION This invention relates to fluid density transducers and more particularly to fluid density transducers in which atubular DESCRIPTION OF PRIOR ART In British Patent specification No. 786] I3 there is described a liquid densitometer in which a hollow cube is connected to two identical tubes which extend coaxially from opposite sides of the cube and are respectively secured in massive clamping blocks. Liquid is passed through the conduit thus formed and the cube is caused electromagnetically to vibrate. The frequency depends on the density of the liquid. Such an arrangement suffers from the disadvantage that it vibrates similarly to 'a loaded beam at its ends vibrating in its fundamental transverse mode so that the securing of the outer ends of the tubes is of critical importance to the attaining of a stable frequency. It is found in practice that satisfactorysecuring of the outer ends of a single tube or conduit vibrating in the' fundamental mode is not practicable.

One solution has been proposed in U.S. Pat. No. 3,444,723. The present invention offers an alternative solution.

SUMMARY OF THE INVENTION In a fluid density transducer according to the present invention there is a tube formed of resilient material, each end of BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of a fluid density transducer, shown partly in section, embodying the invention, and

FIG. 2 is a cross-sectional view of the transducer taken at the line 2-2 in FIG. 1. l

The transducer shown in FIG. 1 is intended for use in mea suring the densities of liquids and includes a cylindrical tube 11 formed of Ni-Span-C (Registered Trade Mark) which is a resilient ferromagnetic metal. Each end of the tube 11 is secured to a respective stainless steel end block 12 which serves to establish a node at the end of the tube 11 for transverse vibration of the tube in operation.

A means for exciting natural transverse vibration of the tube 11 at an overtone frequencyof the tube is provided in the form of a drive and pickup coil assembly 13. The assembly includes a drive coil 14 and two pickup coils 15. Each coil encircles and is supported by the centerpiece of a respective U- shaped magnetic core 16. A respective permanent magnet 16' is secured between the arms of each core 16 to provide a biasing field. The pickup coils serve also as a means for providing a signal representative of the frequency of vibration of the tube excited in operation. The drive coil is coupled to the pickup coils through an amplifier 17 as explained hereinafter.

The tube 11 is of uniform intemal and external diameter through its length. Each end block 12 is cylindrical and has a tapering bore 18 that terminates at its narrow end in an internal flange 19 in which a respective end of the tube 1l fits. The end surfaces of the tube are flush with the respective external surfaces of the internal flanges of the blocks 12. Electron beam welds are effected at the junctions of the ends of the tube and fthe blocks 12 to secure the ends to the blocks.

' The transducer is housed in a rectangular, stainless steel box 20 which is shown in FIG. 1 with one side-removed to reveal the transducer. Flanged pipe connections 21 of the same internal diameter as the tube 11 which are welded into apertures in the end walls of the box 20 are provided. Each'pipe connection 21-is coupled to a respective end block 12 by a respective bellows assembly 22. Each bellows assembly 22 has a stainless steel bellows 23 secured at each end to a short, flanged pipe member 24 bolted either to the block 12 or the pipe connection 21, and a rubber inset 25; The insert 25 has a uniform bore of the same diameter as that of the tube 11.

The end blocks 12 are mounted on supports (not shown) which are secured to a wall of the box 20. These supports are stiff enough in the axial direction of the tube for it to be possible to mount the transducer with the tube vertical substantially without the bellows assemblies 22 being distorted, but not so stiff as to prevent thermal expansion of the tube being accommodated thereby. The supports are substantially rigid transversely of the tube so that they resist any tendency of misalignment of the tube to increase under the pressure of fluid passing through the transducer.

The assembly 13 includes a metal angle piece 26 which is secured to the blocks 12 .by means of two semicircular brackets 27, one at each end of the piece 26. FIG. 2 shows the manner of attachment of one of the brackets 27 to one of the blocks 12 and to the piece 26.

From FIG. 2 it can also be seen that the U-shaped cores 16 of the coils 15 and 16 lie in an axial plane of the tube 11, that is, in a plane containing the axis of the tube.

The positions of the cores 16 longitudinally of the tube, shown in FIG. 1, are chosen to promote'the excitation of the second overtone of natural vibration of the tube 11.

The brackets 27 accommodate thermal expansion of the tube relative to the assembly 13. Buffers 28, each in the form of a nylon screw and locking nut, are provided at the ends of the angle piece 26 to prevent excessive distortion of the brackets 27 by mechanical shocks.

The pickup coils 15 are connected in series with one another across the input terminals of the amplifier 17 by leads 29. The output terminals of the amplifier 17 are connected to the drive coil 14 by leads 30. The leads 29 and 30 are arranged to avoid causing interference with vibration of the tube 11 and transmission of mechanical noise to the assembly 13 from the amplifier 17, which is mounted on a wall of the box 20. Alternatively, the amplifier may be mounted on the angle piece 26.

In operation the liquid of which the density is to be mea sured is passed through the transducer, entering at one pipe connector 21 and leaving at the other. Vibration at the second overtone of natural transverse vibration of the tube 11 is established and maintained by feedback from the pickup coils 15 to the drive coil 14 through the amplifier 17. The phase shift from the-pickup coils to the drive coil is arranged to be such that there is substantially 270 difference between the phase angles of the magnetic flux in the magnetic circuit including the core 16 of the drive coil on the one hand and the magnetic flux in the magnetic circuits respectively including the cores 16 of the pickup coils on the other hand for the frequency of the second overtone of natural transverse vibration of the tube. The permanent magnetic 16 secured to the core 16 of the drive coil eliminates frequency doubling. The position of the core 16 of the drive coil 14 is such that its mag netic field acts on the central antinode of the second harmonic standing wave pattern of the tube, and the positions of the cores 16 of the pickup coils 15 are such that their magnetic fields act respectively on the other two antinodes of this standing wave pattern. The positions required in terms of a fraction of the length of the tube 11 from one end of the tube are, for the drive coil core 0.5, and for the pickup coil cores about 0.2 and 0.8 respectively.

Each pickup coil 15 has induced therein by the transverse vibration motion of the antinodal portion of the tube 11 adjacent thereto an electromotive force which is in phase with the transverse velocity of the said antinodal portion. In order to ensure that the tube 11 vibrates at the resonance value of the second overtone and therefore that the frequency does not vary with damping, the driving force and therefore the magnetic flux exerted on the tube by the drive coil is arranged to be in phase with the transverse velocity of the central antinodal portion. Since the transverse velocity at the central antinode is 180 out of phase with the transverse velocity at the other antinodes, the amplifier 17 is designed to provide a phase shift of 180 from the induced electromotive force in the pickup coils to the magnetic flux of the magnetic circuit linked with the drive coil. The amplifier 17 includes means for adjusting the phase shift produced thereby so that the phase shift can be set to the optimum value. The construction of a suitable amplifier is well known to those skilled in the art and is therefore not further described herein.

The output signal of the amplifier 17 which is fed to the drive coil 14 is also supplied through leads 31 to a frequency meter 32 which, by calibration with standard fluid, reads in fluid density units. The lead 3] is arranged to avoid causing interference with vibration of the tube 11 and, especially if the amplifier 17 is mounted on the angle piece 26, transmission of mechanical noise to the amplifier 17.

The embodiment described is suitable for use in determining the densities of liquids such as aqueous solutions, beer and petroleum. An embodiment constructed as described and having a tube 11 with an internal diameter of 0.900 inch, a wall thickness of 0.037 inch, and a length of 23.16 inches was found suitable for measuring liquid densities of up to 3 gm/cc. The viscosity of liquids such as those mentioned above has no appreciable influence on the frequency of vibration excited.

It can be shown that where f, and p, are constants of the transducer and p, is the density of the fluid in the tube.

An equation for the frequency of transverse vibration is where E is Young's Modulus for the material of the tube,

p. is the mass per unit length ofthe tube,

L is thelength of the tube, and

A is a constant dependent upon the order of the frequency excited and the manner in which the tube is supported,

D being the external diameter of the tube, and

d being the internal diameter of the tube.

It is found that the variation ofA with the manner of support decreases as the frequency excited is changed from the fundamental to overtones of increasingly higher frequency, the diminution in variation being of decreasing magnitude on preceding from the first overtone to the second and so on. A substantial proportion of the diminution in variation takes place between the fundamental and the second overtone.

One test carried out gave the following result:

Value of A Overtones Fuuda- Manner of Support mental 1st 2nd 3rd 4th Free-Free and Fixed-Fired 22.4 61. 7 121 200 298 Hinged-Hinged J. 87 39. 5 88. 9 158 247 Percent Difference in A 56 36 26 21 17 isolate the transducer from the box. Also, the presence of the rubber inserts 25 compensates for a stress in the tube as will now be explained.

When the tube 1] is filled with a fluid having an excess of pressure over the ambient pressure which acts on the exterior of the tube, the excess of pressure tends to alter the length of the tube. An alteration in the length ofthe tube with change in pressure is undesirable since it would result in a variation in the frequency of vibration with variation in the pressure of the fluid. The sense in which the length of the tube tends to alter can be selected by appropriate choices of the dimensions of the tube. The rubber inserts in the bellows assemblies 22 behave as though they were an extension of the body of fluid passing through the tube and since as a result there is a change in effective diameter of the passage for fluid as the fluid passes from a bellows assembly and into the tube, or vice versa, a pressure difference is set up which produces an axial loading of the tube. It can be arranged that the sense of this loading is opposite to that which tends to alter the length of the tube as a result of the pressure of the fluid inside the tube. Consequently by choosing an appropriate mean diameter for the bellows of the assemblies 22 and appropriate dimensions for the tube the effect of the pressure of the fluid in the tube can be substantially nullified.

The use of an overtone as opposed to the fundamental of transverse vibration of a tube considerably diminishes the difficulty of fixing the ends of the tubes without using extremely massive mountings and renders the performance of the transducer less dependent on the nature of the means fixing the ends of the tube. The second overtone harmonic is preferred since it is easy to excite and maintain with simple devices such as drive and pickup coils.

Other embodiments can be constructed in which, for example, the tube is of glass, preferably fused silica, 96 percent sil ica or Pyrex-brand (Trade Mark) glass. The material of the tube should be chosen to have as small a temperature coefficient of expansion as possible over as wide a range of temperature as possible.

Ni-Span-C has satisfactory thermal expansion characteristics for a useful range of temperature. It is an iron-nickelchromium alloy produced by the Huntingdon Alloy Products Division of the lntemational Nickel Company, lncorporated, of Huntingdon, West Virginia and has the following limiting chemical composition:

Nickel (plus cobalt), 4l.043.50% Chromium, 4.905.75

Titanium, 2.202.75%

Aluminum, 0.300.80%

Carbon, 0.06 maximum percent Manganese, 0.08 maximum percent Silicon, 1.00 maximum percent Sulphur, 0.04 maximum percent Phosphorus, 0.04 maximum percent Iron, Remainder.

Further det ails of the properties of Ni-Span-C are given in Technical Bulletin T-3l of the Huntingdon Alloy Products Division.

Means for exciting vibration other than magnetic means may be in the form of, for example, piezoelectric, electrostatic, magnetostrictive, acoustic, or mechanical devices for exciting vibration. Means for providing a signal representative of the frequency excited may be in the form of, for example, optical, electrostatic, piezoelectric, strain gauge, or mechanical devices.

Embodiments of the present invention can be constructed which are suitable for measuring the density of gases. Such an embodiment preferably as a higher instrument sensitivity, that is, change of frequency with change of fluid density, than an embodiment intended for measuring the density of liquids. Instrument sensitivity increases with decreasing wall thickness of the tube and can be further increased by arranging for the gas to flow both through the tube and over its external surface. For example, in an embodiment substantially as shown in the accompanying drawings but having an instrument sensitivity suitable for gas density measurement the two bellows assemblies are omitted and the gas fills the box 20 as well as the tube 11 in operation. In such an arrangement it is desirable for safety reasons and for ease of maintenance that any associated electrical circuitry, for example a maintaining amplifier such as the amplifier 17, be mounted outside any region through which the gas flows in operation. in the modification described above of the embodiment shown in the accompanying drawings filters would be incorporated in the gas flow paths to prevent dirt from causing obstruction in the drive and pickup coil airgaps. 7 7

In general the natural frequency of vibration of a tube is not only influenced by the density of fluid in the tube but is also influenced by the density of fluid surrounding the tube. It a tube is mounted in a case that is filled with air and vented to the atmosphere, the changes in atmospheric density which occur with changes in atmospheric conditions change the natural frequency of vibration of the tube. In an embodiment of the invention constructed in this way the resulting errors in the measurement of densities of the liquids. specifically mentioned hereinbefore are very small. These errors can be eliminated by mounting the tube inside a sealed, evacuated container.

Mounting of the tube in a sealed, evacuated container is also desirable where otherwise the temperature of the fluid occupying the tube would or might be sufficiently low to cause condensation of atmospheric moisture on to the external surface of the tube.

Other advantages of an evacuated container are the thermal insulation provided thereby and the elimination of the small damping effect attributable to the presence of air surrounding the tube. The elimination of this damping effect improves the stability of the instrument and gives a higher 0 factor, that is, the efficiency of vibration is improved.

ln some cases it is desirable to sacrifice some of the advantages associated with the use of an evacuated container for the tube and to mount the tube in a container filled with a dry, low density gas such as helium. This has the advantage that, by equalizing the internal and external fluid pressures acting on the containers walls a pressure differential across those walls can be avoided.

lclaim:

l. A fluid density transducer comprising a right circular cylindrical tube formed of resilient ferromagnetic material,

a pair of end blocks each with a conical hole therethrough,

the blocks being arranged with the holes coaxial and having the wide ends of the holes facing one another,

a pair of internal flanges, one in each of the said holes at the narrow end thereof, the ends of the tubes being located and secured respectively in the apertures of the said internal flanges,

a supporting structure,

a pair of pipe connections fixed in the supporting structure,

a pair of bellows assemblies respectively coupling the ends of the tube to the said pipe connections, each bellows assembly comprising a metal outer sheath member and an inner elastomeric member having a bore aligned with and equal to that of the tube,

mounting means to mount the said end blocks on the supporting structure with compliance axially of the tube but substantially rigidly transversely of the tube,

a drive coil and two pickup coils mounted on a platform resiliently suspended between the said end blocks, and maintaining amplifier means coupling the said pickup coils to the said drive coil, the drive coil being arranged to excite a central antinode in the tube at an overtone frequency and the pickup coils being arranged to sense two other antinodes symmetrically disposed in the tube relative to the said central antinode, the said amplifier means being adapted to provide in operation a vibrationmaintaining phase difference-between the currents flowing through the drive coil on the one hand and the pickup coils 'on the other hand.

2. A fluid density transducer comprising a tube formed of, resilient material, first and second massive clamping means fixedly attached to the ends of said tube for establishing node points defining a vibratable tube portion having a natural resonant frequency f, means for introducing into said tube a fluid the density of which is to be measured, means for exciting natural transverse vibration of said tube at an overtone frequency of at least about 2f, the exact frequency of vibration being representative of the density of said fluid; and means for sensing the resulting frequency of vibration and for displaying said frequency as a measure of fluid densit 3. Apparatus according to claim 2 w erein each of said clamping means comprises a substantially cylindrical body having a central bore greater in diameter than the greatest transverse excursion of said tube, and a radially inwardly extending flange at one end of said bore, said flange defining an inner bore having a diameter equal to the outer diameter of said tube, said flange having an axial thickness significantly smaller than theinner diameter of said tube, said body being end of said tube and said bore surrounding a portion of the vibrating portion of said tube. 

1. A fluid density transducer comprising a right circular cylindrical tube formed of resilient ferromagnetic material, a pair of end blocks each with a conical hole therethrough, the blocks being arranged with the holes coaxial and having the wide ends of the holes facing one another, a pair of internal flanges, one in each of the said holes at the narrow end thereof, the ends of the tubes being located and secured respectively in the apertures of the said internal flanges, a supporting structure, a pair of pipe connections fixed in the supporting structure, a pair of bellows assemblies respectively coupling the ends of the tube to the said pipe connections, each bellows assembly comprising a metal outer sheath member and an inner elastomeric member having a bore aligned with and equal to that of the tube, mounting means to mount the said end blocks on the supporting structure with compliance axially of the tube but substantially rigidly transversely of the tube, a drive coil and two pickup coils mounted on a platform resiliently suspended between the said end blocks, and maintaining amplifier means coupling the said pickup coils to the said drive coil, the drive coil being arranged to excite a central antinode in the tube at an overtone frequency and the pickup coils being arranged to sense two other antinodes symmetrically disposed in the tube relative to the said central antinode, the said amplifier means being adapted to provide in operation a vibration-maintaining phase difference between the currents flowing through the drive coil on the one hand and the pickup coils on the other hand.
 2. A fluid density transducer comprising a tube formed of resilient material, first and second massive clamping means fixedly attached to the ends of said tube for establishing node points defining a vibratable tube portion having a natural resonant frequency f, means for introducing into said tube a fluid the density of which is to be measured, means for exciting natural transverse vibration of said tube at an overtone frequency of at least about 2f, the exact frequency of vibration being representative of the density of said fluid; and means for sensing the resulting frequency of vibration and for displaying said frequency as a measure of fluid density.
 3. Apparatus according to claim 2 wherein each of said clamping means comprises a substantially cylindrical body having a central bore greater in diameter than the greatest transverse excursion of said tube, and a radially inwardly extending flange at one end of said bore, said flange defining an inner bore having a diameter equal to the outer diameter of said tube, said flange having an axial thickness significantly smaller than the inner diameter of said tube, said body being mounted on said tube with said flange being attached to the end of said tube and said bore surrounding a portion of the vibrating portion of said tube. 